1. Field of the Invention
The present invention relates to a control unit for an internal combustion engine of automobiles. In particular, the present invention relates to a control unit for an internal combustion engine, which is equipped with means which is capable of driving an injector of automobile in such a manner that even if the boost circuit in the injector-driving circuit of the control unit goes out of order, the injector is enabled to be stably driven while leaving the circuit structure as it is.
2. Description of the Related Art
With respect to the injector-driving circuit for automobiles, the one described in JP Published Patent Application No. 2003-27994 A has been conventionally known, wherein a valve-opening current control circuit is actuated to transmit an electric current needed for opening the valve to the injector and then a switching element is turned OFF to enable the electric current to moderately fall, after which the switching element is brought into a state of Half-ON by making use of a steep fall control circuit of downstream side and then the electric current is switched to holding current.
Further, JP Published Patent Application No. 2004-124890 A discloses a fuel-feeding apparatus for an engine, wherein the supply of electric current to a solenoid is performed at the time when the theoretical product between a valve-opening signal and a holding signal is materialized, and if the time required for enabling the valve-opening current to reach a predetermined current level counted from the start of the fuel injection pulse is shorter than a predetermined time period, a fuel injection system is judged as being abnormal.
FIG. 1 illustrates a circuit diagram of the driving circuit (for one cylinder) of solenoid coil for the injector in the conventional control unit for an internal combustion engine. This circuit consists of two sections, i.e. one of which being a boost circuit which is constituted by an injector controller 5, a boost coil 1, a diode 2, a switching MOSFET 3, an electrolytic capacitor 4 and a current detecting resistor 6; and the other being an injector driving circuit which is constituted by an injector controller 5, a peak current MOSFET 7, a holding current MOSFET 8, a downstream side MOSFET 11, a reverse current-preventing diode 9, a flywheel diode 12, an injector solenoid coil 10 and a current-detecting resistor 13.
When the driving signals shown at the second, third and fourth rows of FIG. 2 are applied to the peak current MOSFET 7, the holding current MOSFET 8, the downstream side MOSFET 11, respectively, the intensity of current flowing to the injector solenoid coil 10 is monitored by the current-detecting resistor 13 shown in FIG. 1 and then the current intensity thus monitored is fed back to the injector controller 5, thereby enabling an injector current 10-A shown at the first row of FIG. 2 to flow to the injector solenoid coil 10.
In the case of this circuit configuration, there is a problem that if the electrolytic capacitor 4 of boost circuit goes out of order due to the GND short-circuit, the electric current from the holding current MOSFET 8 is caused to backflow to the peak current MOSFET 7, thereby making it impossible to enable electric current to flow to the injector solenoid coil 10 and hence making it impossible to actuate the injector.
Further, when it is judged that the boost circuit has gone out of order, the driving signals shown at the second, third and fourth rows of FIG. 3 are applied to the peak current MOSFET 7, the holding current MOSFET 8, the downstream side MOSFET 11, respectively, thereby making it possible to create a current waveform shown at the first row of FIG. 3. However, there is a problem that in the case of the waveform shown in FIG. 3, since it is constant in current waveform, a maximum current needed for opening the injector is caused to flow all over the entire current conducting region, resulting in an increased heating of the driving circuit, thereby greatly restricting the upper limit of the engine speed.
Furthermore, in the case of this circuit configuration, there is also a problem that although it may be possible, with the addition of Zener diode, to perform a steep fall of electric current, it will lead to a great heat build-up due to the provision of Zener diode, thereby necessitating further restriction of the upper limit of the engine speed.